Design of High Linearity MMIC Power Amplifiers for Space Applications
The high linearity performance in satellite transmitters is receiving increasing attention due to the demands of higher data rates in satellite communication links. During the last decade, digital communication links in satellite system have been replacing the previous analog links increasing the channel efficiency by allocating multiple carriers in the transponder bandwidth. However, multicarrier transponders require high linearity performance in the transmitter. In addition, a highly reliable operation is required for space applications to ensure a lifetime that typically exceeds 15 years. This is achieved by avoiding stress to the device, operating it far below the process maximum ratings and keeping the junction temperature under control.
This thesis presents the design strategy for GaAs pHEMT MMIC high linear power amplifiers intended for multicarrier operation at C and extended C-band. An investigation in junction temperature prediction methods is provided and applied during the amplifier design. A careful device selection method is described with the target to fulfill the electrical and reliability requirements. The circuit design is focused on high linearity. A non-linear transistor model accurately predicts harmonic generation and intermodulation products. This allows optimization of the bias point and output load for high linearity. The amplifiers have been characterized in terms of S-parameters, single tone output power and two tone output power. In addition, infrared temperature measurements have been performed at transistor and chip level.
The circuits are fabricated in a 250nm GaAs pHEMT technology having a cut-off frequency of 45 GHz. A maximum OIP3 of 41.7 dB at 4.5 GHz was measured and gain above 28 dB was observed in an octave band between 3 and 6 GHz. The measured 1dB compression point is 26.7 dBm. The power consumption is less than 2.5 W and measured junction temperature is 112 °C.